CWL: avoid creating snpEff database if missing indices
CWL: avoid creating snpEff database if missing indices

Avoids errors when creating tarballs due to directory nesting:
https://groups.google.com/d/msg/biovalidation/ejKh_hd-mvE/pC3JPRQ5BwAJ

a113n/bcbio-nextgen,vladsaveliev/bcbio-nextgen,vladsaveliev/bcbio-nextgen,vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,chapmanb/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen,chapmanb/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,lbeltrame/bcbio-nextgen